Within the last year we have synthesized 175 peptides for use in 40 separate studies. These studies are run by 29 investigators. The majority of the peptides have been used to prepare antipeptide antisera in rabbits. Antipeptide sera reactive with the C-terminus of the H-2Kb molecule has been used to investigate alternative RNA splicing patterns in various mouse strains and cell types. Antipeptide sera made against the constant region of the alpha, beta, and delta chains of the human T-cell receptor complex is being used to exmine expression and assembly of the complex. Antipeptide sera against the alpha and beta chains of the human class II molecules DR, DQ, DP and DO has been pepared and is being used to investigate expression and association of these molecules. An association of I-A and I-E molecules has been observed in the mouse and the nature of this association is being investigated with antipeptide sera reactive with the I-A and I-E alpha and beta chains. In addition to using peptides to make antisera, peptides have also been used to map determinants recognized by antisera made against intact proteins. This approach has been used for monoclonals against the Plasmodium vivax, Plasmodium berghei and Plasmodium yoelli sporozoite surface antigens. Finally, peptides have been used to define T-cell epitopes in cytochrome C and the plasmodium falciparum sporozoite surface antigen.